FIG. 22 shows a state where a conventional partial discharge sensor is installed on a GIS.
FIG. 23 shows a configuration of a conventional partial discharge sensor.
In the figures, a flange 102a is formed at an end portion of a conductor tube 101a, and a flange 102b is formed at an end portion of a conductor tube 101b. 
An insulating spacer 103 is sandwiched between the flanges 102a and 102b, and holds a high-voltage cable 104 inside the conductor tubes 101a and 101b. 
Connection bolts 105 connect the flanges 102a and 102b with each other.
In this manner, the high-voltage cable 104 is placed inside a tubular closed space formed by the conductor tube 101, and the closed space is filled with a gas that enhances insulation thereof.
The partial discharge sensor 106 is installed on the insulating spacer 103 sandwiched between the flanges 102a and 102b, and transmits a signal detected by the partial discharge sensor 106 to a signal processor 107, to thereby detect a partial discharge that occurs within the GIS.
The partial discharge sensor 106 operates a slot 109 disposed on a conductive casing 108 as a slot antenna.
When an electrical length of the slot antenna that takes a dielectric constant of an elastic member 110 into account is set to a half wavelength of a wavelength to be used, an antenna with high sensitivity can be configured (see Patent Document 1 below).